Math 2.0

statement theorem

Fibonacci numbers as sums of binomial coefficients

Statement

Let nn be a natural number, and let FmF_m be the Fibonacci numbers. Then Fn+1=∑k=0⌊n/2⌋(n−kk).F_{n+1} = \sum_{k=0}^{\lfloor n/2 \rfloor} \binom{n - k}{k}.

Remarks

Example: F6=(50)+(41)+(32)=1+4+3=8F_6 = \binom{5}{0} + \binom{4}{1} + \binom{3}{2} = 1 + 4 + 3 = 8. Combinatorial proof: Fn+1F_{n+1} counts the tilings of a strip of length nn by squares and dominoes. A tiling with kk dominoes has n−kn - k tiles, and (n−kk)\binom{n-k}{k} choices fix the positions of the dominoes. In Pascal's triangle, the sum runs along a shallow diagonal.

Explanations

L1 human

Add the numbers of Pascal's triangle along the shallow diagonals, and you get the Fibonacci numbers. Each term counts the ways to tile a strip with a fixed number of dominoes.

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  1. Binomial coefficient definition
  2. Fibonacci sequence example
  3. Fibonacci numbers as sums of binomial coefficients statement

Sources

  1. Web https://en.wikipedia.org/wiki/Fibonacci_sequence, Combinatorial identities